The present invention relates to a test apparatus for a mixing apparatus for multi-component plastics, in particular polyurethane, having a first valve in a component return line and a second valve in a test line which branches off the return line upstream of the first valve, the two valves being operated jointly and in the opposite sense with respect to one another.
In a known test apparatus of the above type, the two valves are each formed by a ball valve. The ball valves can be actuated jointly by means of a pneumatic actuator. An outlet pipe for smoothing the flow of the component to be tested before the latter flows into a test beaker or the like is located downstream of the ball valve in the test line. The flow-smoothing outlet pipe is necessary because the ball valve releases only a very small cross-section at the beginning of its opening movement, with the result that very agitated, ie. squirting, flow is produced. Out of the test amount bled off, a larger or smaller unknown residual amount remains in the outlet pipe, falsifying the final test result. This applies in particular to the isocyanate component, which crystallizes out in the outlet pipe, ie. is deposited there, so that this pipe may even have to be replaced. The two ball valves forming the valves are connected so that negative overlap is present, ie. during opening and closing there is a transition range in which both paths are open. The component to be tested is divided up in an undefined manner, which may likewise lead to errors in testing. In particular, the ball valve constituting the first valve in the test line is impeded in its movement by component material which has crystallized out and has been deposited on the outer surface of the ball as well as in the outlet pipe. The result is that movement is impeded, at least during the initial actuation after a fairly long shutdown time, so that the intended test time cannot be maintained. This in turn leads to an error since the test amount per unit time must be established. In the case of pronounced crystallization, this ball valve may no longer open at all. Furthermore, the crystals result in greater wear.